The invention relates to a method for performing a diversity combination in a communications system.
In radio communications systems, messages and information, for example voice, image information or other data, are transmitted using electromagnetic waves between the transmitting and receiving station (base station and subscriber station respectively) via a radio interface. In existing mobile radio communication networks conforming to the GSM standard (GSM: Global System for Mobile Communication), new data services such as a packet data service GPRS (General Packet Radio Service) are also currently being introduced. In addition to the transmission of voice data, third-generation communications systems, e.g. the UMTS (Universal Mobile Telecommunication System) in accordance with the UTRA standard (UTRA: Universal Terrestrial Radio Access), also provide for the transmission of packet data units (PDUs). The packet data units are derived by segmenting and by adding additional control information from large data packets of higher layers or system levels (e.g. Layer 3). In particular, packet data is transmitted asynchronously, or not synchronously, with the result that the transmission durations and/or the transmission paths of individual packet data units transmitted in succession may differ from one other. To identify the packet data units arriving at the receiver, the units are provided with packet data identification information or sequence number. In the systems currently used or proposed, the identification information is transmitted in the header section (header) of the data packet or as transport format combination identifiers (TFCI).
The correct transmission of the identification information is very important here since the data packets are later frequently combined into large blocks, and incorrect identification information would lead to an incorrect combination of individual data packets. In the worst case, such an incorrect combination is not detected until very late, or even not at all, so that a renewed request to transmit the packet data is issued very late, or even incorrect data are further processed. In particular, a non-detected processing of incorrect identification information leads to serious disruption of the traffic at air interfaces between the transmitter and receiver, for the most part with long-lasting consequences.
Since data losses can occur in a multiplicity of situations during the transmission of the packet data units, data protection methods for transmitted data are known. In particular data protection includes encoding methods and repetition methods, e.g. an automatic data repetition method with combinable encoding for forward error correction (FEC) known for short as Hybrid ARQ Type I or II (ARQ: Automatic Repeat Request), which are sketched in FIG. 1. Following a first unsuccessful data transmission of a packet data unit PDU, a renewed transmission is requested (ACK/NACK) by the receiving station MS from the transmitting station BS. While the first transmission can be performed with optional encoding (P1), encoded data P2 are transmitted at least for the repetition. In this case the redundancy can increase from repetition to repetition, with a corresponding increase in the probability that the data packets PDU can be correctly reconstructed at the receiving end.
FIG. 1 shows three ways of obtaining corrected data. The error corrections that are based directly on only the received polynomials P1 and P2 correspond to the ARQ-I method, in which different polynomials P1 and P2 need not necessarily be selected. With the ARQ-I method, the receiver does not need to store the data from earlier transmissions in order to decode the current transmission. The error correction shown in the middle of FIG. 1 combines or chains in accordance with the ARQ-II method the data of the two received polynomials P1 and P2. If the two polynomials P1 and P2 are identical, the encoded data can be combined, which is referred to as diversity combining. If the two polynomials P1 and P2 are different on the other hand, the receiver cannot combine the associated transmissions, but should chain them to form a combined “larger” encoding block, which is subsequently decoded. With diversity combining, in the case of the incorrect reception of a plurality of differently encoded data polynomials, the transmission of the data polynomial Pi that was received with the lowest signal-to-noise ratio is preferably requested again.
In connection with the hybrid ARQ error correction, a diversity combination can be used, e.g. a maximum ratio combining (MRC), if copies of earlier transmitted encoded data of the same polynomial are repeated. With increasing redundancy transmission, that is to say with the increasing number of repeated transmissions with stronger encoding in each case, however, the following errors can occur in the receiving station:
1. If the signaling information is transmitted in-band in the header section of the transmitted data block, that is to say together with the payload data, and is not protected here with an individual checksum (CRC: Cyclic Redundancy Check) to save system capacities and hence costs, then undetected errors can occur when decoding the header section. If the identification information of the data packet or of the header section was corrupted or lost, then the corresponding data block (with diversity combining or chaining) is processed with data from other data packets or data blocks not belonging to it. In particular, this can lead to the destruction of valuable buffered baseband information in the receiving station. In the worst case, such errors can even propagate unnoticed, or only noticed at a late stage.
2. If the receiving station attempts to decode a data block transmitted by an alien transmitting station and received as a result of an overreach, comparable problems may arise.
In general it can be observed that as a rule the above methods offer good results if the noise signals superimposed on a data block can be described as Gaussian distributed noise. In mobile radio communications, however, signaling of alien transmitting stations or data transmitted over other paths can lead to interference that does not correspond to any Gaussian distribution. Such interference cannot be readily handled using methods such as the maximum ratio combination MRC. In particular outliers that are superimposed on the wanted receive signal with high amplitudes also make a negative impact as noise signals. One specific problem of the maximum ratio combination MRC is the necessity of estimating the signal-to-noise ratios from the transmitted data prior to the actual combining. It is no trivial matter to obtain a stable estimate of signal-to-noise ratios when there are outliers present in the transmitted data.
A code combining in which a code rate is adapted to prevailing channel conditions is known from the publication “Cide Combining—A Maximum Likelihood Decoding Approach for Combining an Arbitrary of Noisy Packets”, David Chase, IEEE Transactions on Communications, IEEE INC. New York, US, VOL.COM-33, No. 5, May 1985, pages 385–393.
A data transmission in which a data packet with errors is transmitted again is known from WO 99/26371.